dc42:
polymorph:
As far as using "add a resistor to drive a MOSFET Gate" rule-of-thumb, if you are using this with PWM, you'll make MOSFET heating even worse by extending the time spent in the linear region.Only if you are using a combination of high pwm frequency (in which case you should definitely use a series resistor when driving a power mosfet) and too high a value series resistor. If a 100 ohm series resistor is too high to let the mosfet run cool, then you should be using a mosfet driver chip.
The rule of Ib=Ic/10 to saturate a bjt is a reflection of the fact that for most bjts, the datasheet doesn't guarantee that you can get a low saturation voltage with anything less, because Vce(sat) is quoted at that ratio and hfe is quoted at a relatively high Vce such as 10V.
Yes! Exactly the point, you must first understand much more before applying a rule of thumb, in order to understand where it is and isn't appropriate.
For instance, for a quick-and-dirty circuit using generic BJT transistors (2N2222, 2N3904, 2N3906, etc.), I liberally apply the Ib = Ic/10 or 20 rule-of-thumb. For a final circuit, I use known transistors and read the datasheet.